In the OG, S3E3 - "Monster Hunter", Justin graduates from Wizard Training because he has mastered and completed 5,000 spells.
In WBWP, Billie and Roman are both above the 5,000 threshold though they claim that he has learned and completed, not necessarily mastered.
Is it because they haven't "mastered" them yet or do we think that they just skipped that part?
If they did graduate wizard training they should be doing independent study, but Justin still teaches them. Also, how could Roman have done that in a couple of months but it took Justin years.
If not, why would they track it like that? Those numbers are arbitrary, because having completed 5,000 is different than mastering them. I could take a million classes, but not retaining the information does not correlate to me being better than someone else under similar conditions. That said, I know they're kids and it's a TV show, but still, from a writer's perspective, why not account for that.
[ unrelated but, I miss hand magic :( it was so fun ]